Generally, this invention relates to an electrophotographic printing machine, and more particularly concerns an adjustable screen being periodically cleaned to remove particle contaminants therefrom.
In a typical elecrophotographic printing machine, a photoconductive member is charged to a substantially uniform potential. The charged photoconductive member is exposed to a light image of an original document. This records an electrostatic latent image on the photoconductive member corresponding substantially to that of the original document. The electrostatic latent image is then developed with charged particles. A sheet of support material is positioned closely adjacent to the latent image and arranged to have the particles transferred thereto. After the particles are transferred to the sheet of support material, they are permanently affixed thereto forming a copy of the original document. Generally, the charged particles are heat settable so that the application of heat thereto permanently secures them to the sheet of support material.
Multi-color electrophotographic printing incorporates the features of the foregoing process. However, a plurality of single color light images are created. Successive single color light images are formed by filtering the light image of the original document. The single color light image irradiates the charged photoconductive member recording thereon an electrostatic latent image corresponding to a single color contained in the original document. A plurality of different single color electrostatic latent images are formed and developed with toner particles complementary in color to the color of the filtered light image. These powder images are transferred to the sheet of support material in superimposed registration with one another. The multi-layer toner powder image is then permanently affixed to the sheet of support material by the application of heat thereto.
Hereinbefore, it has been difficult to reproduce subtle variations of tone or color. To overcome this difficulty, half-tone screens have been employed. These screens produce tone gradations by forming half-tone dots or lines of varying size. In the highlight zones, for example, the dots are small. These dots increase in size throughout the intermediate shades until they merge together in the shadow region. At the extremes, there will be complete whiteness at the highlight end of the tone scale, and solid black at the shadow end. Numerous patents teach the concept of screening. Exemplary of these patents are U.S. Pat. Nos. 2,598,732; 3,535,036; 3,121,010, 3,493,381; 3,776,633; and 3,809,555. However, these patents do not, as a rule, teach cleaning of the screen to remove particle contaminants therefrom.
It is the primary object of the present invention to improve the cleaning of a screen employed in an electrophotographic printing machine.